1. Field of the Invention
The present invention relates to a submarine optical cable resistant to a longitudinal flow of water accidentally penetrated inside it and to a method for controlling said longitudinal flow of water inside said cable.
Submarine optical cables are subjected, in case of accidental rupture thereof, to a sudden ingress of a sea-water flow at high pressure (e.g. 100 bar, when the cable is at 1000 m below the sea level). Such high pressure water flow may propagate for a relevant length inside the cable if suitable water blocking means are not provided in the cable, thus damaging a remarkable portion of said cable which has then to be replaced.
2. Description of the Related Art
A number of cables designed for submarine installation are known in the art.
For instance, U.S. Pat. No. 5,125,062 discloses an undersea cable comprising a central metallic tube, filled with a sealing compound, e.g. silica gel, and containing optical fibers embedded therein, said tube being surrounded by a helical lay of metallic (preferably steel) wires. Interstices between wires and between the helical lay and the central tube are filled with a sealing material (preferably polyurethane resin) which opposes longitudinal propagation of water along the cable. Alternatively, the central tube can be made of plastic and in this case the helical lay also presents the characteristics of an arch for withstanding pressure.
U.S. Pat. No. 4,684,213 relates to a submarine cable comprising a pressure resistant steel tube containing optical fibers, surrounded by two layers of steel wires and by an outer metal tube made of copper or aluminum. Dams of a sticky compound and/or of a jelly of plastic material are disposed at regular intervals inside the central tube and in the gaps between the lay of wires disposed between the central tube and the outer tube.
U.S. Pat. No. 5,463,711 discloses an underwater cable for shallow-water, comprising a central tube made of plastic or preferably of metal, optical fibers arranged within said tube and surrounded by a water blocking material and six steel wires wound in a helical lay around the central tube.
UK patent application no. 2,253,717 describes a method for manufacturing an optical cable in which a plurality of optical fibers are enclosed as they are fed in a longitudinal direction by a continuously fed metal strip folded around the fibers; the edges of said metal strip are welded together to form a tubular moisture barrier, the diameter of which, at the welding stage, provides sufficient clearance between the fibers and the heat affected zone of the welded tube. The diameter of the welded tube is then reduced by passages through the one or more sets of reducing rollers, the tube then closely surrounding the optical fire package. The diameter reduction increases the tensile strength of the metal tube.
In general, for submarine cables apt to operate at deep depths (e.g. below 500 meters) it is required that the water penetration length along the cable is less than 1000 meters after 2 weeks.
Applicant has now observed that when a tube (in particular a buffer tube containing optical fibers) is of relatively small internal diameter (e.g. below about 7 mm), it is rather difficult to achieve a complete filling of said tube with a suitable water-blocking gel. As a matter of fact, because of the relatively small diameter of buffer tubes specifically adapted for submarine cables (typically of about 2-5 mm) and of the relatively high viscosity of the filling jelly compound (generally from about 50 Paxc2x7s to about 150 Paxc2x7s), the filling of the tube can only be completed for about 80-95% of the internal volume of the tube. In addition, an incomplete filling of the buffer tube may be desirable in some instances, also for buffer tubes of larger dimensions. For instance, as disclosed in European Patent Application Publ. No. EP 883007, if optical fibers are exposed to hydrogen gas, the transmission properties thereof are altered, the higher being the partial pressure of hydrogen the more relevant said alterations. EP 883007 thus suggests filling only partially the buffer tube with a filling material (maximum 95%), in order to leave an expansion volume inside the tube so that the partial pressure of hydrogen is kept relatively low.
Thus, for different reasons, from about 5 to about 20% of the internal volume of the buffer tubes can be left free from water blocking gel material.
The applicant has now observed that, while this incomplete filling can be considered of relatively low importance for terrestrial cables (where longitudinal water penetration tests are conducted under a water head of 1 m), it becomes much more important for underwater cables. As a matter of fact, Applicant has observed that the voids due to the incomplete filling tend to dispose longitudinally along the whole length of the buffer tube, thus creating a preferential path along which a high pressure water flow (e.g. 100 bars if the cable is laid at a depth of 1000 m) can flow with relatively low head losses for a relevant length of the cable before being blocked.
In addition, if water has a preferential flow path inside the buffer tubes, the respective hydrostatic pressure may act on the inner surface of the plastic tube, thus increasing the diameter of the same; as a consequence, the flow path is increased in dimension and the water flow increases the speed and the penetration length inside the buffer tube.
Applicant has now found that by providing a suitably dimensioned longitudinal cavity (or xe2x80x9cgapxe2x80x9d) defined along the outer surface of a buffer tube containing optical fibers, said buffer tube being made of a deformable material (e.g. polymeric material), it is possible to substantially reduce the longitudinal flow of water inside said buffer tube. For instance said buffer tube can be surrounded by a second tube made of a substantially non-deformable material (e.g. metal) having a larger inner diameter with respect to the outer diameter of the buffer tube, so to create the desired gap around the buffer tube. In particular, the Applicant has observed that the dimensions of the longitudinal space surrounding the buffer tube should be selected in order to provide along said space a preferential flow path for the water accidentally penetrated inside the cable, with respect to the flow path inside the buffer tube, thus preventing water from flowing inside said buffer tube. In other terms, the head losses of the water flowing along said longitudinal space surrounding the buffer tube should be lower than the head losses of the water flowing inside said buffer tube.
One aspect of the present invention thus relates to a method for controlling a longitudinal flow of water accidentally penetrated inside the structure of a submarine cable, said cable comprising a deformable elongated hollow body comprising at least one optical fiber loosely housed therein, wherein said method comprises causing the water to flow inside the cable, but outside said deformable elongated hollow body, for a distance longer than the distance of the water flowing inside said deformable elongated hollow body.
In particular, the water flow through said deformable elongated hollow body has a first head loss, while the water flow inside the cable, but outside said deformable elongated hollow body, has a second head loss, said second head loss being lower than said first head loss.
Preferably, said method comprises the steps of:
disposing within said cable structure a first elongated hollow body defining an inner surface;
disposing within the inner surface of said first elongated hollow body an optical core comprising at least a second deformable elongated hollow body comprising at least one optical fiber loosely housed therein;
defining a longitudinal cavity inside said first elongated hollow body and outside said deformable elongated hollow body;
disposing a water blocking element within said space
causing the water to flow with a lower head loss through said cavity than through said deformable elongated hollow body.
Preferably, said first elongated hollow body is a substantially non-deformable elongated hollow body.
Advantageously, the head losses of the water flow along said longitudinal space can be lower than about 75% of the head losses of the water flow inside the deformable elongated hollow body, preferably lower than about 50%, much preferably lower than about 30%.
According to a preferred embodiment, water is caused to flow through a longitudinal space of predetermined dimension between the inner surface of said substantially non-deformable elongated hollow body and the outer surface of said optical core.
Another aspect of the present invention relates to a submarine optical cable comprising:
at least a first elongated hollow body defining an inner surface;
at least one optical core housed within said first elongated hollow body, said optical core comprising at least a second deformable elongated hollow body;
at least one optical fiber loosely housed in said second deformable elongated hollow body;
a longitudinal cavity defined inside said first elongated hollow body and outside said deformable elongated hollow body;
a waterblocking element disposed within said cavity;
wherein:
a water flow through said first deformable elongated hollow body has a first head loss;
a water flow through said longitudinal cavity has a second head loss;
said second head loss being lower than said first head loss.
Preferably, said first elongated hollow body is a substantially non-deformable elongated hollow body.
According to a preferred embodiment, the cross-sectional area defined by the inner surface of the non-deformable elongated hollow body is greater than the cross-sectional area defined by the outer surface of the optical core, so that a longitudinal cavity between the inner surface of the non-deformable elongated hollow body and the outer surface of the optical core is defined, through which water can flow with less head losses than inside the elongated hollow body.
In the present description, the term xe2x80x9coptical corexe2x80x9d is meant to relate to the inner structure of an optic cable, wherein optical fibers are disposed. In particular, said optical core may comprise a single central deformable buffer tube containing the optical fibers housed therein or may comprise a number of deformable buffer tubes disposed with a helical lay around a central reinforcing element. Said optical core may further optionally comprise yarns and/or tapes (e.g. a waterswellable tape) wrapped around the central buffer tube or around the group of stranded buffer tubes.
The term xe2x80x9csubstantially non-deformable elongated hollow bodyxe2x80x9d refers in the present description to an elongated hollow body, e.g. a tube or an annular helical lay of wires presenting arch characteristics, which is dimensioned so as to sustain the hydrostatic pressure of the water at the laying depth of the cable without undergoing substantial structural deformation.
According to a preferred embodiment, said substantially non-deformable elongated hollow body is a metal tube, e.g. of copper, steel or aluminum, having a predetermined thickness, elastic modulus and yield strength so as to resist to a selected hydrostatic pressure. Such a metallic tube provides an hermetic protection towards radial penetration of water or gases into the inner structure of the cable where the optical core is located and can be used, in particular if it is made of copper, for supplying electric power to the system components. As an alternative, said substantially non-deformable elongated hollow body can be formed by an annular lay of metal wires, e.g. of steel, preferably wound as a helical lay around the optical core, said helical lay presenting the characteristics of an arch for withstanding the relevant hydrostatic pressure of the selected depth. In this latter case, the annular lay of metal wires can be in turn surrounded by a metal tube, e.g. of copper or aluminum, capable of hermetically protecting the inner structure of the cable from radial penetration of water or gases.
According to a preferred embodiment, the inner diameter of said metallic tube should preferably exceed the outer diameter of the optical core of from about 0.2 mm to about 1.5 mm, much preferably of from about 0.4 to about 1.0 mm, a difference of about 0.5-0.6 mm between the two diameter being particularly preferred.
The term xe2x80x9cdeformable elongated hollow bodyxe2x80x9d refers to an elongated hollow body, e.g. a tube, capable of being deformed, in particular compressed, by the hydrostatic pressure caused by the water on its outer surface at the selected depth of lay. Typically said tube is filled for less than about 95% of its volume, e.g. from about 80% to about 95%, with a waterblocking filling composition.
Preferably, said deformable elongated hollow body is a tube having one or more layers of polymeric material, for instance polyester (e.g. polybutyleneterephtalate), polyolefin (e.g. polyethylene, polypropylene, copolymer ethylene-propylene), or polyamide, having a predetermined thickness and elastic modulus so that when a hydrostatic pressure of a selected degree is applied on the outer surface of said tube (as a consequence of the water penetration at the relevant laying depth of the cable), the tube is sufficiently compressed so as to reduce the void volume inside the mass of the filling material, thus reducing the distance covered by the water flow. Advantageously, the compression of said tube should be such as to reduce its original cross-sectional area of at least about 3%, preferably of at least 5%, when subjected to the relevant hydrostatic pressure.
According to an embodiment of the present invention, said deformable elongated hollow body is a polymeric buffer tube disposed in a helical lay around a central supporting member.
According to an alternative embodiment, said elongated hollow body is a single plastic tube coaxially disposed within said non-deformable elongated hollow body.
The water blocking element disposed within the longitudinal cavity along the deformable elongated hollow body should be such as to allow the water penetrating in said longitudinal passage to flow with less head losses than inside the deformable elongated hollow body at the initial stage of the penetration while allowing said head losses to gradually increase in time, eventually blocking the flow of water in said longitudinal passage within a predetermined length of the cable from the inlet point of the water.
In general, the block of water is intended to be effective if water penetrates inside the cable for a distance of less than 1000 meters in two weeks.
Preferably, said water blocking element is a water blocking tape which is wrapped around the optical core or around the deformable hollow body. When said deformable hollow body is a polymeric buffer tube disposed in a helical lay around a central supporting member, the water blocking tape may be wrapped around the buffer tubes already stranded around the central support member.
Alternatively, said water blocking element can be a filling material, e.g. an elastomer or a jelly-like composition, discontinuously disposed along the longitudinal direction of said longitudinal passage.